In the past, various processes have been devised for coating metal surfaces with latex by inserting the metal part to be coated into an aqueous bath containing a fine dispersion of latex particles. By using the metal part as an electrode and by causing an electrical current to flow through the bath, the charged latex particles will be attracted to and will adhere to the surface of the metal part. Electrical processes of this type, known as electrophoretic processes, are relatively complex and costly.
Similar processes for depositing coating on metals from aqueous media in the absence of applied electrical currents are known as chemiphoretic processes.
One chemiphoretic process which has been used to form latex coatings employs an aqueous bath containing both hydrofluoric acid and hydrogen peroxide (See German Offenlegungsschrift No. 22 11 490 dated Sept. 21, 1972 and German Offenlegungsschrift No. 24 09 876 dated Sept. 26, 1974). In this process, an electrical current is not required in order to form a latex film; however hydrofluoric acid is a dangerous chemical and excessive metal is dissolved during the coating process. Furthermore, a relatively long time, on the order of 2 minutes, is required to form a 1-mil film; and the build-up of salts in the bath due to the highly corrosive hydrofluoric acid rapidly reduces its effectiveness.
A similar chemiphoretic process is described in U.S. Pat. No. 3,585,084 and in Prieve et al, Ind. Eng. Chem. Prod. Res. Dev., 17, Number 1, 1978, Pages 32-36.
Another chemiphoretic process for coating metal articles with latex is described in German Offenlegungsschrift No. 26 23 895 dated Dec. 9, 1976. In this process, a metal object, preferably a steel object, is brought into contact with an aqueous cationic dispersion of latex particles, the aqueous dispersion containing a carboxylic acid radical such as the carboxylic radical of mandelic acid or citric acid. The application is primarily concerned with the production of cationic latexes by emulsion polymerization of monomers such as vinyl esters of fatty acids with 1 to 18 carbon atoms, unsaturated acids such as acrylic acid, esters of unsaturated acids with alcohol, glycols or epoxides with 1 to 18 carbon atoms, acrylonitrile or styrene, 1,3-butadiene. The German application implies that special cationically stabilized latexes are required in order to carry out the coating process.